1. Field of the Invention
The present invention relates to digitally encoded television transmission systems, and, more particularly, to systems detecting NTSC signals.
2. Description of the Related Art
In the United States, the Advanced Television Systems Committee (ATSC) has proposed a digital television standard for High Definition Television (HDTV) transmission systems. A typical transmitter 100 and receiver 120 of an ATSC transmission system is shown in FIG. 1. The transmitter 100 comprises a video encoder 102 for compressing digital video signals, an encoder & trellis coder 104 for Reed-Solomon coding and trellis coding the signal from video encoder, a precoder 106 for preceding the signal output from encoder & trellis coder 104. Precoding by precoder 106 combines selected symbols of the data stream in a manner that is reversed by an NTSC filter in the receiver 120, thereby canceling NTSC interference as described subsequently. The transmitter 100 also comprises a modulator & SAW filter 108 for forming the signal output from precoder 106 into a form of vestigial side band within 6 MHz, and a radio frequency (RF) transmitter 110 for transmitting the signal from modulator & SAW filter 108 through an RF channel 112.
The receiver 120 comprises a radio frequency (RF) tuner 121 including an intermediate frequency (IF) surface acoustic wave (SAW) filter for selecting a RF channel and providing an IF signal. The IF signal is provided to a demodulator 122 to provide a baseband signal, known as an I-channel signal, and timing recovery circuit 123 recovers data clock, synchronization and timing clock signals from the I-channel signal containing composite symbols for data and timing. The demodulator 122 also may include a synchronous detector and analog-to digital converter (not shown) which provides the I-channel signal as digital samples. An NTSC detector and rejection filter 124, which may be a comb-filter and controller, detects and cancels NTSC co-channel interference in the baseband I-channel signal. A channel equalizer 125 compensates for distortion of the I-channel signal by the RF channel 110 and distortion of the comb-filter, if used, of NTSC detection and rejection filter 124. The I-channel data symbols of the compensated I-channel signal are then applied to a bit de-interleaver (not shown) and error correction and trellis decoding circuitry 126 which performs Reed-Solomon decoding and trellis decoding of the I-channel data symbols to form a decoded bit stream. The decoded bit stream from the error correction and trellis decoding circuitry 126 is then reformatted to a digital data stream by deformatter 128. Deformatter 128 reformats the decoded bit stream since the original digital data stream of an encoder is formatted so as to appear as a random bit stream. The reformatted digital data stream is then decoded by video decoder 130 to provide video signals.
NTSC interference rejection is based on the frequency location of the NTSC co-channel interfering components with respect to transmitted HDTV signals, which relationships are illustrated in FIGS. 2A-2C. FIG. 2A illustrates a RF spectrum of a HDTV signal as transmitted. FIG. 2B illustrates a RF spectrum of an NTSC signal that may cause co-channel interference. FIG. 2C illustrates frequency characteristics of a comb filter as typically used to remove NTCS co-channel interference.
As shown in FIG. 2B, the NTSC signal includes picture carrier, color sub-carrier and audio carrier signals. The comb filter frequency characteristics have null points spaced 896.85 kHz apart which null points are around the frequencies of the picture carrier, color sub-carrier and audio carrier signals. Passing the NTSC signal through a comb filter having such characteristics removes these carrier signals. FIG. 3 is a block diagram of a conventional NTSC comb filter 300. As shown in FIG. 3, the filter 300 may be a single tap, feed forward filter and comprises a delay 301 and subtractor 302. Delay 301 provides a delayed I-channel signal, to subtractor 302, and delay 301 typically delays the I-channel symbols by 12 symbols. Since the comb-filter forms a difference of a symbol and a delayed symbol, the precoder 106 of the transmitter anticipates the comb-filtering and adjusts each symbol accordingly.
The conventional NTSC comb-filter 300 as shown in FIG. 3, while providing rejection of steady state signals at null frequencies has a finite response of, for example, 12 symbols. In addition, while the comb filter reduces NTSC co-channel interference, the data is also modified. As a result of the single tap filter forming a difference of two full gain paths, the comb filter decreases signal-to-noise ratio, degrading white noise performance by 3 dB. Consequently, the ATSC transmission system only comb-filters when necessary. Therefore, an ATSC receiver 120 includes an NTSC detector that only enables NTSC filtering and equalizes the baseband signal when the presence of the NTSC signal is detected.
These NTSC detectors of the prior art typically monitor the signal energies of the un-filtered and filtered baseband signals, and only enable the NTSC comb filter when a SNR drop of greater than 3dB occurs. When an NTSC signal is not present in the baseband signal, filtering doubles the noise power, or reduces SNR by 3 dB, in the filtered signal. A minimum energy detector, therefore, may be used to compare interference noise power, u.sup.2, of the baseband signal with the interference noise power, f.sup.2, of the filtered baseband signal. If u.sup.2 is greater than f.sup.2 /2, then the NTSC signal is present and filtering is enabled.
Since the I-channel signal includes both a data component (data symbols) and timing component (data field sync signal), an NTSC detector of the NTSC detection and rejection filter 104 of FIG. 1 typically measures a signal-to-interference plus channel noise ratio of the data field sync signal path. This measurement is typically performed by creating and comparing two error signals. The first error signal is created by comparing the received signal with a stored reference of the data field sync signal, and the second error signal is created by comparing the comb-filtered data field sync signal with a comb-filtered version of the data field reference signal. Consequently, the NTSC detector includes a second NTSC filter which comb filters the extracted data field sync signal.